Well, the question is: How do you know who is invited?
What qualifies an "invited" user?
An easy approch would be to have a "send invitation" script accessible for your members, which would take the email-address of a new member, generate a random invitationid and send a link to the reg-from (including this id) to the email-address.
The script register.php must then check if there is an invitationid and if it is valid, otherwise it must reject registration
here is how it works on our site (its small 21 members at this point)
there is a group of 6 of us we pick someone we want to invite on the board and one of us sends the person the link. I then ofcourse allow them in via admincp.
so the invite would be sent via a admin or at the minimun a moderator. so i think the invite should be part of the admin/mod control panel. now there is a validation email that is sent, couldnt a invite email be sent also? now that would be more like what i was think of.